In recent years, it has been known that propagation characteristics of electromagnetic waves can be controlled by periodically arranging a conductor pattern having a specific structure (hereinafter, referred to as a metamaterial). Particularly, a metamaterial constructed to suppress propagation of electromagnetic waves in a specific frequency band is referred to as an electromagnetic bandgap structure (hereinafter, referred to as an EBG structure). The EBG structure is used as a countermeasure against noise propagating in a circuit board or the like.
An example of such a technique is described in Patent Document 1 (U.S. Pat. No. 6,262,495). FIG. 2 of Patent Document 1 shows a structure, that is, a mushroom-like EBG structure, in which plural island-like conductor elements are arranged over a sheet-like conductive plane and the respective island-like conductor elements are connected to the conductive plane through vias.